Compression
Technology for the Next Generation

Danny
Deffenbaugh (left), director of SwRI’s Mechanical and Fluids Engineering
Department in SwRI’s Mechanical and Materials Engineering Division,
manages the Advanced Reciprocating Compression Technology program. Program
Manager Dr. Klaus Brun (center) is the principal developer of the
semi-active electromagnetic plate valve for the ARCT program, and Staff
Engineer Dr. Ralph Harris (right), conceived of the tapered cylinder
nozzle design. Brun and Harris are also staff members in the Mechanical
and Fluids Engineering Department.

The
natural gas industry’s transmission system remains one of the vital energy
infrastructures in the United States. This industry operates pipelines
throughout the country, with compressor stations at regular intervals of 30 to
100 miles to help ensure a steady flow of gas. More than 70 percent of the
compressor fleet, or approximately 4,000 units, are reciprocating-piston
compressors.

Advances in
compression technology helped the U.S. gas industry expand after World War II.
The original, first-generation compression technology consisted of many small
(500 to 750 horsepower) slow-speed (180 rpm) compressors to move gas from
producing regions to markets. To provide the necessary expansion, a
developmental second generation of larger, higher-speed machines promised
significant reductions in installed costs. However, as industry installed the
first of these machines they experienced many reliability and operational
problems involving flow pulsations and mechanical vibrations that resulted in
piping failures.

To address these
problems, the pipeline industry in 1952 formed what is now the Gas Machinery
Research Council (GMRC), which contracted with Southwest Research Institute
(SwRI). As a result of this initial collaboration, SwRI developed pulsation
control systems that combined acoustic filters and dampers with effective
mechanical restraints. SwRI has continuously operated the GMRC pulsation design
service for decades, generating royalties that have funded GMRC research since
1955.

This second generation
of compression technology has since become known as “slow-speed integral”
compression. This equipment has nominally three times the horsepower, running at
twice the speed (at less than 300 rpm in the 1,500 to 2,500 hp range) of the
equipment it replaced. These slow-speed integral machines have been the U.S.
pipeline industry’s compression workhorses.

The promise of
dramatic cost reductions has driven the industry toward even higher-speed,
larger-horsepower reciprocating compression, powered by efficient, separate,
modern gas engines or large electric motors. Within the past few years, the
first iteration of this new class of machines has been installed. This third
generation of equipment has three times the power of the prior generation and is
now running at two to three times the speed (500 to 1,000 rpm; 4,000 to 8,000
hp). With this technology have come new vibration and pulsation problems. The
pipeline industry faces a technology transition similar to that of 50 years ago.
As a few large machines replace many small ones, each must provide a wider
flow-rate capacity range with increased reliability. Wider variations in speed
complicate pulsation control, and higher speeds have resulted in significant
losses in compressor efficiency, contributed to in part by both pulsation
control and conventional valve technology.

Slow-speed, integral
machines are generally no longer commercially available because they are
cost-prohibitive to manufacture and install. While affordable, the current
high-horsepower, high-speed compressors require advancements in technology to
meet their full potential to address the pipeline industry’s compression needs.

Slow-Speed Compression
180 rpm, 500 to 750 hp

Slow-Speed Integral Compression
300 rpm, 1,500 to 2,500 hp

High-Speed Separable Compression
500 to 1,000 rpm, 4,000 to 8,000 hp

Compression technology has advanced
since World War II from small, slow-speed compressors (left), to the faster
integral compressors developed in the mid 1950s (center) to today’s
high-speed, high-horsepower machines (right).

Advanced
Reciprocating Compression Technology (ARCT) program

To meet growing
demands for energy, the U.S. Department of Energy initiated a Natural Gas
Infrastructure program with the goal of increasing capacity of the current
pipeline infrastructure by 10 percent and reducing operational costs by 50
percent. Under funding from the DOE Office of Fossil Energy, National Energy
Technology Laboratory-Delivery Reliability Program, SwRI led an effort in
conjunction with the GMRC to formulate a research program for DOE to address
this challenge. The objective of the ARCT program is to create the next
generation of reciprocating compressor technology to enhance the flexibility,
efficiency, reliability and integrity of pipeline operations. The suite of
technologies developed during this program will not only provide pipeline
operators with improved, affordable choices for new compression, but will also
provide innovative solutions that can be retrofitted to existing machines to
substantially improve current reciprocating compression.

The primary challenges
for the slow-speed, integral fleet are limited flexibility, large range of
thermal performance, and significant operating and maintenance costs. The
primary challenges for the new, high-speed compressors are cylinder nozzle
pulsations, mechanical cylinder vibrations, short valve life and even lower
thermal efficiency. The goals for next-generation compression are improved
flexibility (50 percent turndown in flow rate), improved efficiency (more than
90 percent), improved reliability and maintenance (increase valve life by a
factor of 10 with half the pressure loss) and improved integrity (vibration
levels less than 0.75 inches per second).

The initial ARCT
program was a five-year, three-phase program, which began in October 2004. Many
promising technologies were developed at SwRI during the first phase of the
program, completed in October 2005. Two particularly significant ones were a
tapered nozzle pulsation control device and a semi-active electromagnetic valve.

SwRI engineers developed a tapered
cylinder nozzle for high-speed compressors that decreases both pulsation
amplitude and pressure loss.

This comparison of a standard nozzle
(red) and the tapered cylinder nozzle design (green) documents a 64 percent
reduction in pulsation amplitude and a shift in frequency from 48 to 72 Hz.

Pulsation Control

The state of the art
in pulsation design and control technology has evolved as compressor technology
installed by industry has changed. Designs for low-speed compressors are more
mature, with fewer critical issues. However, relatively recent high-speed,
high-horsepower compressor designs are placing significant challenges on the
pulsation control designer.

Cylinder nozzle
response represents the single most important challenge to high-horsepower,
high-speed, variable-speed units. Significant reductions in unit efficiency and
capacity occur through use of pressure drop elements required to control
pulsation amplitudes. Technology is required to allow control of the nozzle
response without significantly lowering cylinder performance. This is
particularly important if compressor flexibility (in turndown ratio) is
required, or if the trend toward higher speed continues.

For high-speed
compressors, there is a need to lengthen the cylinder nozzles (to reduce
mechanical coupling), raise the resonance frequency and reduce pulsations at the
cylinder nozzle resonance frequency. SwRI engineers formulated a concept to
replace conventional, straight compressor cylinder nozzles with tapered cylinder
nozzles. The tapered cylinder nozzle concept lowers the effective acoustic
resistance, thereby reducing the acoustic reflection, decreasing both the
resultant pulsation amplitude and the pressure loss through the nozzle. Benefits
associated with this concept include significant increase in the cylinder nozzle
resonant frequency and lower amplitudes of excitation. Also, it allows for
lengthening the cylinder nozzle such that mechanical coupling between the
cylinder and the rest of the piping is reduced.

Simulation and
experimental data of the tapered cylinder nozzle show both a significant shift
in the cylinder nozzle resonant frequency and reduced amplitude of the cylinder
nozzle pulsations. At the same time, the associated thermal efficiency is
improved due to the reduced parasitic pressure loss through the nozzles.

Results demonstrated
that the cylinder nozzle resonant frequency shifted above the fourth order of
running speed or 24 to 26 Hz frequency, which is a 50-percent increase. The
maximum nozzle pulsation amplitudes were reduced by 34 to 35 percent, and the
pressure drop was reduced by one-third. The tapered cylinder nozzle results
showed fewer pulsations (below 3 percent of line pressure) with less pressure
drop than would be required in a traditional system with a straight nozzle
installed. (The American Petroleum Institute pulsations guideline is 7 percent
of line pressure for these operating conditions.)

The tapered cylinder nozzle reduces
pulsation level and parasitic pressure loss. The top curve shows pulsation
response and pressure loss for a standard nozzle, while the bottom curve
shows the reduced pulsation level and pressure loss for the tapered nozzle.

SwRI researchers
concluded that the tapered cylinder nozzle is a viable concept that warrants
further development. This concept has demonstrated the potential to resolve the
critical cylinder nozzle problem experienced in modern high-speed compression
and may very well be the needed enabling technology for next-generation
compression.

The financial benefit
to reducing pressure loss in these nozzles can be realized either in terms of
improved thermal efficiency or in expanding the flow through the station. At a
current cost of natural gas of $9 per thousand cubic feet, a 6 percent
improvement in overall thermal efficiency would result in a cost savings of
$50,000 per year per compressor.

The second approach to
realizing financial benefit from an improvement in efficiency is to increase gas
flow throughput and avoid the need to install new compression. The estimated
capital cost reduction can vary over a large range. A conservative estimate is
to assume a $1,000 per horsepower benefit and calculate a simple capital cost
savings. The same 6 percent efficiency improvement for a slow-speed integral
would recover $120,000 in capital cost.

While this technology
development has undergone a proof-of-concept experiment, additional development
is required before full-scale acceptance by the industry can be expected.
Laboratory testing of the technology, along with development of design tools, is
under way, and will be followed by prototype demonstration in an actual pipeline
under realistic conditions. SwRI engineers can then incorporate the proven
technology in the designs of advanced pulsation control systems.

The semi-active electromagnetic
plate valve is a relatively simple modification to a standard plate valve,
with the addition of a center shaft and a speaker voice coil.

Compressor Valves

The single largest
maintenance cost for a reciprocating compressor is compressor valves. Valve
failures can primarily be attributed to high-cycle fatigue, sticking of the
valve, accumulation of dirt and debris, improper lubrication and liquid slugs in
the gas. Valves are designed for an optimal operation point; hence, valve
operation is impaired when the operating conditions deviate significantly from
the design point. In the traditional compressor valve design, an increase in
valve life (reliability) directly relates to a decrease in valve efficiency.
This relationship is due to an increase in valve lift (and flow-through area)
being limited by the corresponding increase in the valve impact force. Above a
certain impact velocity, valve plate failure is attributable to plastic
deformation of the valve springs. These springs fail to provide adequate damping
for the plate. The design of the valve springs is a major weakness in the valves
currently in use. A lack of durability and low efficiency of the passive valve
design demonstrates the need to control valve motion.

Reducing impact
velocity can greatly increase the life of a valve. SwRI engineers have developed
a new valve concept that could create a soft landing at both the valve seat on
closing and at the valve guard on opening. The concept is to effectively replace
the valve spring with an electromagnetic coil that senses position and provides
an opposing force prior to impact. This concept is referred to as a “semi-active
electromagnetic plate valve” because it is still activated by gas pressure and
only controlled prior to impact. This new valve concept was initially tested
using a single impact shock tube. During this testing, a valve element was
coupled to the voice coil of a speaker to provide a variable reaction force. The
reaction force applied by the coil was able to measurably reduce the impact
velocities of the valve plate. The reduction in impact velocities resulted in a
relative life gain of 3 to 11 times that of a standard valve experiencing the
same forces. Engineers designed and implemented a full-scale test breadboard in
a reciprocating compressor located at SwRI. The design included a standard valve
configuration, with modifications to couple it to an electromagnetic voice coil.

The foundation of the
semi-active electromagnetic plate valve is similar to existing plate valves in
service today. Only slight modifications are needed to facilitate the addition
of the voice coils. Initial testing showed that the valve profile becomes
rounded, thus reducing the impact velocity. Reduced impact velocities result in
a significant increase in valve life, since high cycle fatigue is the primary
cause of valve failures. The financial benefit of this valve technology is based
on the increased life and the associated reduced maintenance cost for valve
change-out. However, the new valve technology will require some additional cost
per change-out. The valve life improvement is a factor of 10 and the increased
cost is a factor of two. If, for example, the current life is a half year and
change-out cost is $30,000, the overall maintenance cost benefit is calculated
to be $240,000 over a five-year period, or a normalized annual savings of
$48,000.

The SwRI valve design
has proven to be worth further evaluation. The next step is to reduce the size
and complexity of the unit, investigate a self-powering feature and eventually
field test the unit in actual pipeline compressors.

The SwRI-developed semi-active
electromagnetic plate valve design allows for a soft landing of the valve,
thereby extending its life by 3 to 11 times that of standard valves. Valve
replacement is a significant cost for the gas industry.

Other ARCT
Developments

During the course of
Phase 1 of this project, other successes included 18 additional technology
solutions. These technologies have been developed to a proof-of-concept stage.
The industry has recommended advancing half of these technologies to the next
stage. A conservative estimate for the value of this suite of technologies is
$50,000 per installation per year.

For the future,
solutions are needed for both slow-speed and high-speed compression. For
slow-speed compression, optimum flow rate turndown will be accomplished with a
combination of speed and clearance. Advances in pulsation control will recover
capacity lost due to pressure drop. Advances in valves will extend valve life
with low-pressure loss penalty. The combination of these technology improvements
will provide the potential of 95 percent thermal efficiency with three-year
valve life and expanded flow rate turndown.

For high-speed
compression, optimum flow rate turndown will be accomplished with unit speed.
Tapered cylinder nozzles will resolve the nozzle pulsation problem with half the
pressure loss and also eliminate cylinder vibration. An additional new
technology developed during this program is a tunable side-branch-absorber that
addresses the fundamental frequency vibration in the lateral piping over the
entire speed range with minimal pressure loss penalty. The semi-active
electromagnetic plate valve will extend valve life with half the pressure loss
penalty. The combination of these technology improvements will provide the
potential of 90 percent thermal efficiency with two-year valve life, 50 percent
flow rate turndown and vibrations of less than 0.75 inches per second.

SwRI engineers believe
that the program will develop sufficient technology solutions to address the
current limitations of modern high-speed compression, thus enabling this
equipment to meet its full potential. If so, this program will meet its stated
objective of creating the next generation of reciprocating compressor technology
that provides added pipeline flexibility at reduced costs.